The present invention meets these needs by providing improved methods of filling gaps. In certain embodiments, the methods involve placing a substrate into a reaction chamber and introducing a vapor phase silicon-containing compound and oxidant into the chamber. Reactor conditions are controlled so that the silicon-containing compound and the oxidant are made to react and condense onto the substrate. The chemical reaction causes the formation of a flowable film, in some instances containing Si—OH, Si—H and Si—O bonds. The flowable film fills gaps on the substrates. The flowable film is then converted into a silicon oxide film, for example by plasma or thermal annealing. The methods of this invention may be used to fill high aspect ratio gaps, including gaps having aspect ratios ranging from 3:1 to 10:1.